Penultimate unit effects in the free-radical copolymerization of styrene with acrylonitrile were investigated by the consideration of the theoretical thermochemistry of three subsequent propagation steps in the copolymerization process at 298 K and the electronic properties of the relevant reactants. The total energies, zero-point energies incorporating a 0.96 scale factor, and thermal enthalpy corrections for all optimized structures were computed with the B3LYP density functional theory and the 6-311G(d,p) basis set. The penultimate unit effect on the enthalpy of reaction for elementary propagation reactions ranged from -1.2 to 2.7 kcal/mol. The enthalpies of elementary copolymerization propagation reactions showed that penultimate unit effects depended not only on the gamma substituent itself but also on the terminal unit of the growing radical and the monomer being attached. The exothermicity of the addition of radicals, varying in the penultimate unit for a given monomer, was lower for more polar radicals and smaller Mulliken charges at the radical atom, except for radicals with a CN substituent placed in the gamma position when they reacted with acrylonitrile. For the latter system, the repulsive interactions between the CN substituent and nitrile group of the monomer being added contributed to the reaction enthalpy. Almost no penultimate unit effect was detected upon spin distribution at the radical atom, and this probably indicated the absence of the independent implicit penultimate model. The results obtained strengthen the concept of the inseparability of implicit and explicit penultimate unit effects in radical copolymerization. However, it appears that for the styrene-acrylonitrile copolymerization system, the explicit penultimate model prevails over the implicit penultimate model.